(a) Field of the Invention
The invention concerns a heat treatment of metals in a continuous oven by continuous longitudinal passage of metallic pieces in an elongated treatment zone under controlled atmosphere having an upstream end at elevated temperature where said controlled atmosphere comprises nitrogen and reducing chemical substances, such as hydrogen, possibly carbon monoxide, and a downstream end under an atmosphere essentially formed by introducing nitrogen.
(b) Description of Prior Art
This type of controlled atmosphere which is essentially utilized for annealing metallic pieces is up to now produced in the following manner:
either by utilizing an exothermic generator which is responsible for the incomplete combustion of a hydrocarbon and air and produces combustion gases which, possibly after purification, contain hydrogen and carbon monoxide in amounts which depend on the air/hydrocarbon ratio introduced into the generator. By way of example, such an exothermic atmosphere may contain 5 to 10% carbon monoxide and 6 to 12% hydrogen;
or there is produced a synthetic atmosphere from pure industrial gases such as nitrogen and hydrogen. The nitrogen is produced by cryogenic distillation of air and contains very little impurities; for example, the total amount of water vapor and oxygen impurities is generally lower than 10 vpm. To this highly pure nitrogen, hydrogen, or a hydrocarbon, or hydrogen and hydrocarbon, or methanol are added so as to produce a reducing atmosphere, which may be non decarburizing, to treat the metallic pieces.
This second procedure has the advantage of completely controlling the treating atmosphere but has the disadvantage of utilizing cryogenic nitrogen which is relatively costly and consequently inadequately adapted for use in generally non-impervious continuous ovens. This is the reason why attempts have been made to reduce the flows of gases introduced by creating for example at the outlet of the cooling zone a nitrogen buffer which enables to prevent any upward introduction of air through the cooling zone thereby ensuring a significant reduction of the global flow of gas introduced. In spite of this important reduction of the global flow, it has been found that the pure industrial gases are still far from being economically attractive as compared to gases which are produced in an exothermic generator.
This is the reason why, in certain applications where this has been found possible, it has been proposed to replace cryogenic nitrogen by nitrogen produced by air separation according to the techniques of adsorption or selective permeation which, under certain conditions of production, substantially reduce costs as compared to cryogenic nitrogen. This procedure is however to the detriment of the oxygen impurities since nitrogen produced by adsorption usually contains a residual amount of 0.5% to 5% oxygen while the residual content of oxygen in nitrogen produced by permeation generally exceeds 3% and may reach up to 10%.
This oxygen impurity makes it very difficult to use raw nitrogen directly to prepare a suitable atmosphere for the heat treatment. In practice, it has been proposed to use nitrogen produced by the selective permeation process only for the production of atmospheres prepared from nitrogen and methanol, as described in the article "Heat treating processes with nitrogen and methanol based atmosphere" M. KOSTELITZ et al., in "Journal of Heat Treating" volume 2, No. 1-35 and in U.S. Pat. No. 4,279,406 and EP-A-0213011. Such an atmosphere prepared from nitrogen having a residual content of oxygen and methanol can indeed be theoretically used for certain applications, namely heating before hardening, carbonitridation and cementation of steel. However, it is only in this last mentioned field of application that nitrogen with a residual amount of oxygen has been used on an industrial basis and this is because of the elevated temperature that is required for cementation, which is of the order of 900.degree. C., this temperature promoting the reaction of residual oxygen carried by nitrogen with the chemical substances of the hydrocarbon type which are simultaneously introduced to constitute the basic atmosphere.
It has been suggested to purify nitrogen with a residual content of oxygen produced by adsorption or permeation, by catalytically reacting oxygen with a corresponding input of hydrogen which is sufficient to lead to the complete elimination of any oxygen, but this process which is relatively costly implies a production cost which is close to that of cryogenic nitrogen, which goes against this method of preparation of pure nitrogen, inasmuch as the production of nitrogen by adsorption or permeation does not have the advantages of flexibility and simplicity as the production of cryogenic nitrogen.